Drug users caught in possession of cannabis on the streets three times within a year will be arrested under a "three spliffs and you're out" rule announced by police chiefs yesterday.

But the charity DrugScope claimed the move could mean "conservative police elements" were trying to frustrate ministers' intentions and hang on to their powers.

The new advice on possession is contained in a policy document drawn up by the Association of Chief Police Officers and revealed in part at the association's drugs conference in Blackpool. Those caught with cannabis once or twice in 12 months will be warned. If caught a third time they will be arrested and could be cautioned or face a court.

"We want to be tolerant but we don't want to be too liberal," said Andy Hayman, deputy assistant commissioner with the Metropolitan police and chairman of Acpo's drugs sub-committee.

He said Acpo felt an arrest at the time of a second offence "would not be tolerant", but to do nothing after three offences would be too liberal.

DrugScope's chief executive, Roger Howard, said the Home Office had given assurances that the power of arrest would only be used in exceptional circumstances where aggravating factors were present.

"Repeat personal possession is not, in our view, an aggravating factor," he said. "This three strikes policy is likely to hit those communities who are subject to the most stop and searches. Cannabis law will therefore continue to be a source of friction with particular communities."

Other provisions in the Acpo document are intended to plug a gap which emerged during the Lambeth experiment in south London on a softer approach to cannabis. Those caught openly smoking will be arrested and taken to a police station if they refuse to hand over their drugs.

The guidelines are about to go out to public consultation and could come into effect after Christmas, six months before cannabis is likely to be reduced from class B to C status following an announcement by the home secretary, David Blunkett, last July.

Speaking at the conference, Home Office minister Bob Ainsworth said the new classification of cannabis would be policed in a way that was not "resource intensive".

He added: "In most cases a warning will be sufficient. But where there are aggravating factors, the police will retain power of arrest."

During the conference police chiefs also warned of the need for more research on crack cocaine, and Mr Ainsworth said a new strategy on the drug would be announced in coming months.

· An investigation into a series of newspaper allegations, including smoking cannabis, against the Lambeth police commander Brian Paddick has been completed, it emerged yesterday, writes Nick Hopkins.

The deputy chief constable of Humberside, Gordon Clark, has sent his report about the claims to the police complaints authority. The report will now be forwarded to the Metropolitan police authority. The MPA will convene a disciplinary committee within weeks to decide what action, if any, should be taken against Mr Paddick.